This "reverting plus sign" behavior usually indicates a handshake failure between your local Music app and the iCloud Music Library servers. The app thinks it added the album, but the server rejects the change or fails to acknowledge it. Here is how to fix it:
1. Refresh "Sync Library" — Go to Settings > Music and toggle "Sync Library" (or "iCloud Music Library") OFF. Wait 30 seconds, then toggle it back ON. This forces a fresh sync of your library's manifest.
2. Sign out and back into Media & Purchases — Go to Settings > [Your Name] > Media & Purchases > Sign Out. Then sign back in. This refreshes your account's subscription tokens and often resolves library modification errors.
3. Check your iPhone Storage — If your device is extremely low on storage (less than 1GB free), iOS may prevent the Music app from writing new metadata to the library database. Check this in Settings > General > iPhone Storage.
4. Check Apple Music Subscription — Ensure your subscription is still active in Settings > [Your Name] > Subscriptions. If there is a billing issue, the library will become "read-only" and you won't be able to add new content.
5. Force Restart the iPhone — Press and quickly release Volume Up, then Volume Down, then hold the Side button until the Apple logo appears. This can clear minor OS-level glitches affecting app background tasks.
6. Try adding via a different network — If you're on Wi-Fi, try switching to Cellular (or vice versa). Sometimes specific network configurations or DNS settings can block the specific Apple server responsible for library updates.
Hope this stops the albums from un-adding themselves!